Hide/Lock History dropdown menu

See #135 (comment 840319832)

This dropdown

is a bit misleading because Use custom settings for history is the only option that makes sense for LibreWolf. Users could be tempted pretty quickly to select Never remember history which is bad because

1. It enables private mode which means all user installed add-ons will be disabled and the browser becomes more fingerprintable
2. It overwrites the settings which cookies to keep

So the idea would be to lock this setting to custom (that should be privacy.history.custom to true) and then optionally hide the dropdown or change the strings in it.